This invention relates generally to position and orientation tracking and, in particular, to methods and apparatus for accurate position, orientation and motion-tracking within a bounded volume in the presence of electromagnetic distortion.
Existing electromagnetic tracking systems, as well as inertial and combined inertial/optical and optical/magnetic tracers, are sensitive to various kinds of distortion. With respect to electromagnetic trackers, such distortion may arise from eddy currents in metal objects or from ferromagnetic materials, whereas, in the case of inertial trackers, drift or vibration might be the cause.
In inertial tracking systems, for example, as described in U.S. Pat. No. 5,645,077, requires an additional sensor, or set of sensors, to compensate for drift and movement of a vehicle or aircraft reference frame. Even with these additional sensors, such systems exhibit sensitivity to vibration, temperature instability requiring additional compensation. Inertial tracking systems also experience drift over time periods on the order of minutes to hours.
Combination systems, that is, systems which combine optical and magnetic sensing, are designed to compensate for distortion by comparison of the data from two different types of sensors. One such system is described in U.S. Pat. No. 5,831,260 to Hansen. The use of combination systems with an optical tracking module (e. g., inertial/optical or magnetic/optical) is restricted to applications where interference associated with night-vision devices is significant, or when parasitic illumination or optical noise is present.
In an electromagnetic tracking environment, distortion may arise from eddy currents induced in nearby metal objects or from ferromagnetic materials. Eddy currents, in turn, generate fields that interfere with the field from the source(s) used for tracking purposes. To compensate for distortion of this kind, one solution involves the use of mapping. With mapping, the electromagnetic field in a volume of interest, as distorted by metal objects, is defined in advance and used to solve for position and orientation. Commonly assigned U.S. patent application Ser. No. 09/215,052, the entire contents of which are incorporated herein by reference, discloses, as part of a preferred embodiment, the use of Green""s functions in conjunction with such field mapping.
The need remains, therefore, for a simple but effective approach to reducing the effects of distortion in an electromagnetic tracking system. Ideally, such a solution would be useful in a variety of applications, including military, motion capture and medical instrumentation.
The subject invention is directed to distortion compensation in electromagnetic position and orientation tracking configurations. Among the advantages over previous approaches, the invention eliminates the need for mapping in advance of actual measurements and, permits real-time adjustment in the event that new field distorters arise. This may be significant in such an environment as aircraft cockpit or tracing of medical/surgery instruments.
The invention is specifically applicable to position/orientation tracking systems of the type wherein the components of an AC electromagnetic field are sensed within a bounded volume. As in previous designs, one or more probe sensors are placed on an object being tracked within the volume, each measuring the magnetic induction vector components of the field generated by the source to determine the position, orientation and movement of the object within the volume. However, to compensate for electromagnetic distortion, the invention employs it least one stationary sensor, termed a witness sensor, in addition to the probe sensor(s) disposed on the object. Each witness sensor is supported at a known, fixed position and orientation relative to the reference frame of interest, at a point near or within the volume of interest, and close to the sensors on the object being tracked.
The outputs of each probe sensor, and each witness sensor, are delivered to a processing unit operative to compute the position and orientation of the object in the presence of electromagnetic field distorters. The processor uses data from each witness sensor to compute parameters, such as the position, orientation, and strength, of an effective electromagnetic source or sources. The effective source(s), which may be treated for the sake of simplicity as a point source (wherein the size of the source is negligible in the scale of measured distances) or a dipole (wherein the field is described by the dipole equation), would produce the same field as a superposition of the real source plus field of distortion in the proximity of witness sensors. Note that in the case of a non-distorted quiet environment, the single effective source will coincide with, and will be identical to, the actual real source. This allows the computed parameters of the effective source(s) to be used as inputs to the computation of position and orientation as measured by each probe sensor, as if the object is in the non-distorted electromagnetic field produced by the effective source(s).
An electromagnetic position/orientation tracking system according to the invention therefore preferably includes a real source of an AC electromagnetic field driven by the system; at least one witness sensor measuring electromagnetic induction vector components at a known spatial point with respect to the real source of an AC electromagnetic field; one or more probe sensors measuring electromagnetic induction vector components from the perspective of the object; and a control/processing unit providing the requisite computations. In addition to trackers for helmet-mounted displays in aircraft, tank, and armored-vehicle applications, the invention finds utility in any electromagnetic tracking system which might be subject to electromagnetic distortion or interference. Such application areas include electromagnetic motion capture systems, and medical systems and instruments, among others.